The present disclosure is directed to methods of making a polymer, including exposing a reaction mixture including a strained cyclic unsaturated monomer and an organic initiator to a stimulus to provide an activated organic initiator, whereby the activated organic initiator is effective to polymerize the strained cyclic unsaturated monomer via a 4-membered carbocyclic intermediate to provide a polymer having constitutional units derived from the strained cyclic unsaturated monomer.